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Full text of "Handbook Of Chemical Engineering - I"

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56
CHEMICAL ENGINEERING
Three-phase, Three-wire.—Three single-phase circuits, the return currents neutralizing. Balanced when the phases are equally loaded.
Three-phase, Four-wire.—The fourth wire a neutral: lamps connected between any outside wire and a neutral wire* Rarely used: six-wire system also in infrequent use.
RELATIVE VOLTAGES AND RELATIVE WEIGHTS or COPPEK (FENDER)
System	Relative Voltages between outer wires		Relative weight of copper1	
	Neutral grounded	Neutral insulated	Neutral grounded	Neutral insulated
Direct-current, two-wire . . ^ », fa Single-phase, two-wire . . . *a |1 • Two-phase, four-wire. . . . £ o,^ Three-phase, three-wire . .	100 71 71 61	50.0 35.5 35.5 35.5	100 200 200 200	400 800 800 600
1 Not including neutral wires.
Primary Supply.—In large works, with alternating current locally generated, primary voltages of 2,000 to 2,400 are common. For distribution to substations, 5,000 to 110,000 volts may be used. The losses in transmission are partly constant (corona, core loss of transformers, parts of the losses of all rotating apparatus, meter losses and losses at constant-current devices, if there are any); and partly variable with the load, usually varying as the square of the current carried.
Voltage Regulation.—Loading tends to pull down voltage, especially toward the outer ends of lines. The extreme drop at maximum load is limited as already described. Fluctuations due to bad regulation may introduce further drop, decrease of motor output and efficiency, decrease in life and illuminating power of lamps, etc. The extreme voltage variation on lighting circuits is rarely allowed to exceed 5 per cent: on power circuits it may be 10 per cent and is sometimes more. In three-wire or alternating-current systems, the best voltage condition is obtained when the loads on the circuits are equal (or are those for which the circuits were designed). The motor-generator used on a three-wire system takes current from the lightly loaded side and delivers current to the other side. The cost of operation is that due to the losses in the machine and the neutral current need not be taken back to the generator. In alternating-current systems (single-phase three-wire) the balancing coil may be used. This consists of two transformer windings connected in series across the outside wires. The neutral is connected from between the two windings. The lightly loaded side acts to excite its winding as a transformer primary, and current is thereby transferred by way of the other winding to the other side of the circuit. In polyphase systems, the phases are of course balanced (so far as power applications are concerned) by using polyphase motors.
Electric Lighting.-—The value of lights is measured in candle-power. The candlepower of a light varies according to the direction from which it is viewed. If C = candlepower in a stated direction which makes an angle a with a plane to be illuminated, the distance from the light to this plane being I ft., then illumination in candle feet = E = C cos a/l\ The candle feet required for